20 May 2011

Birds dislike tannin in vineyards

It is not just humans that enjoy the fruit of the vine. Birds also like the subtle flavours, the delicate nose and varying sweetness of the grape. Unfortunately, this has lead to conflict between grape-growers and birds. Small songbirds, like blackbirds, starling and silvereyes, are pest species in the vineyard and cause millions of dollars of damage each growing season and force growers to spend millions more on protecting their crop. A long-term series of studies at Lincoln University by Val Saxton has the aim of understanding what it is about grapes that birds like and dislike which will in turn provide possible strategies for reducing bird damage. Val has already looked at the influence of sugar, aroma and organic acids on the decision of blackbirds and silvereyes to eat grapes. In a study just published at Austral Ecology she turns her gaze to important compounds within the grape: tannins (used in plant defence and UV protection) and anthocyanins (used in forming pigments or colour). Tannins are in high concentration in ripening grapes and are known to cause digestive stress in some birds when eaten. Anthocyanins signal the stage of ripening that a grape has reached.

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Val made use of an experimental technique that she has developed where she creates artificial grapes out of gelatine, filled with sugar water. Shecan then alter the levels of tannins precisely and, in a second experiment, change the colour (green or purple) without there being any other differences between the test 'grapes'. Birds were offered the test grapes on racks set out in vineyards and their feeding decisions were videoed. Blackbirds and silvereyes are common vineyard pests in New Zealand and their behaviour was recorded and compared. Both species were found to dislike tannins, although silvereyes could tolerate higher levels than blackbirds. In summer, there was no preference by the birds for either colour. However, in autumn, the time of grape harvest, blackbirds took only purple grapes and silvereyes green. Wild grapes are purple and a vivid purple is a signal that the grapes are ripe. Unlike blackbirds, which swallow grapes whole, silvereyes peck at grapes and suck out the fluid. This allows silvereyes to avoid ingesting grape skin and seeds which contain the bulk of tannins in grapes and probably accounts for the greater tolerance to tannins by this species. Overall, this research continues to build a picture about the difference between bird species and a greater understanding of how birds make decisions within the vineyard.